Vp Anomalies Research Articles

Local earthquake tomography of the Aegean crust: Implications for active deformation, large earthquakes, and arc volcanism

Three-dimensional velocity models can enable accurate earthquake location, improved seismic hazard assessment, and can enhance our understanding of geodynamic processes. This is particularly true for areas such as the Aegean, where the crust is marked by active volcanoes as well as shear and rift zones, all of which make this region highly heterogeneous. This work describes the application of local earthquake tomography in order to elucidate the crustal structure of the Aegean. We utilized a dataset of 2,135,625 P arrivals and 1,095,515 S arrivals from 178,676 events recorded across 324 stations. Results indicate that the boundary separating the slow Vp from the fast anomalies at 20–40 km depth is well correlated with the Aegean Moho obtained from receiver functions. A slow Vp (−5%) zone at depths >30 km along the Hellenic arc is likely associated with basal underplating that forms the base of the crust. On the other hand, fast Vp and moderate to high Vp/Vs (1.77–1.92) in the upper-middle crust may indicate evolving metamorphic core complexes. Steeply dipping clusters of earthquakes could highlight pathways of fluid migration from the base to the middle/upper crust, albeit more detailed seismological studies are needed to confirm this interpretation. The 3D velocity model can be also utilized in order to investigate the amount of melt fraction beneath active volcanoes and the influence of fluids on the rupture zone of large earthquakes. Our results show a melt fraction between 4 and 10% beneath active volcanoes, with the largest volume of melt present beneath Santorini caldera. The ruptures zones of 10 large earthquakes (Mw ≥ 6.0), show slow Vp anomalies and moderate to high Vp/Vs (> 1.77), suggesting that these events were likely triggered by weakening of their source zones by fluid activity.

Seismic properties of mantle metasomatism from mantle xenoliths beneath the North Tanzania Divergence, East African Rift

We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6–80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80–90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040–1200°C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2–4 % to 28–37 % for Vp and by 2–3 % to 25–29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %–CO2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between −9 % and −15 %) and vein-bearing peridotites (depth 40–90 km) corresponds to 12–25 % of crystallized veins or 8–15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD.

Subsurface anatomy of the Irazú–Turrialba volcanic complex, inferred from the integration of local and ambient seismic tomographic methods

SUMMARY Irazú and Turrialba are a twin volcanic complex that marks a distinct stop in volcanism along the Central America volcanic arc. We present a new traveltime velocity model of the crust beneath Irazú and Turrialba volcanoes, Costa Rica, and interpret it considering the results of previous ambient noise tomographic inversions. Data were acquired by a temporary seismic network during a period of low activity of the Irazú–Turrialba volcanic complex in 2018–2019. Beneath the Irazú volcano, we observe low P-wave velocities (VP = 5 km s−1) and low velocity ratios (VP/VS = 1.6). In contrast, below the Turrialba volcano, we observe low S-wave velocities (VS = 3 km s−1) and a high VP/VS (= 1.85) anomaly. We found that locations of low VP and VS anomalies (−15 %) correspond well with shear wave velocity anomalies retrieved from ambient noise tomography. At shallower depths, we observe high VP and VS anomalies (+15 %) located between the summits of the volcanoes. Subvertical velocity anomalies are also observed at greater depths, with high VP and VS anomalies appearing at the lower limits of our models. We propose a complex structure of an intermediate magmatic reservoir, presenting multiphase fluid states of a liquid-to-gas transition beneath Irazú and a juvenile store of magmatic fluid beneath Turrialba, while shallow fluid transport provides evidence of magmatic–hydrothermal interactions.

Seismic imaging of the Java subduction zone: New insight into arc volcanism and seismogenesis

The Java subduction zone is located in the southern part of Southeast Asia, where the strong plate convergence results in clustered arc volcanoes and intense seismicity. To better understand the arc magmatism and seismogenesis in this region, we determine a high-resolution 3-D P-wave velocity (Vp) model of the crust and upper mantle down to 600 km depth beneath Java. The geometry of the subducting Australian slab is taken into account in the starting model to obtain a better tomographic result by inverting a large number of travel-time data of local and teleseismic events. Our results show lower intra-slab Vp anomalies at depths of ∼100–200 km than other parts of the slab, which may reflect dehydration embrittlement and a slab hole. Feeble low-Vp anomalies appear below the subducting Australian slab, which is probably due to the slab hole where subslab hot materials flow into the mantle wedge. This feature may be related to the occurrence of potassium-rich back-arc volcanoes and the lack of giant megathrust earthquakes (Mw ≥ 8.5) in Java. Our tomographic results also support the hot finger model for explaining the arc magmatism in Java.

Sound Velocities of Superhydrous Phase B up to 21 GPa and 900 K

AbstractSound velocities and density of Mg10Si3H4O18 superhydrous phase B (SuB) were investigated by synchrotron X‐rays combined with ultrasonic interferometry techniques in a multi‐anvil apparatus, up to 21 GPa and 900 K. Our data show that SuB has compressional (VP) and shear velocities (VS) lower than major anhydrous minerals but higher than majorite garnet. Our data combined with those of other mantle minerals allowed estimation of VP and VS of hydrated mantle regions at depths of the mantle transition zone and upper lower mantle. The local hydration of pyrolitic mantle with SuB as the main water host could explain low VP and low VS anomalies near cold subduction zones while it would be associated with low VP and high VS anomalies in hot mantle regions. The anomalies may be particularly strong in the ∼30 km beneath the 660 km discontinuity, due to the discrepancy of post‐spinel transition between hydrous and dry pyrolite lithologies.

Crustal Compositional Variations From Continental to Oceanic Domain: A VP/VS Ratio Study Across the Zhongsha Block, South China Sea

AbstractThe VP/VS ratio is an important property for understanding magmatic and tectonic processes at passive continental margins as it is an indicator of the crustal composition. To classify the dominant lithologies in the Zhongsha Block, South China Sea (SCS), we present a detailed VP/VS crustal model based on the independent tomographic inversion of P wave and S wave data. The average VP/VS in the crust of the Zhongsha Block is ∼1.77, indicating an overall felsic to intermediate composition lacking remnant magmatic intrusive rocks. The VP‐density relationship from gravity modeling suggests that the lower crust of the extended continental domain contains more greenschist and hence may have experienced metamorphism resulting from an elevated geotherm in the Northwest Sub‐basin either during the syn‐spreading or postspreading stage. The variability of the VP/VS ratio in the continental block is larger than that in the oceanic basin, showing distinct crustal properties. Several low VP/VS ratio anomalies (VP/VS < 1.7) were found near tectonic boundaries and are interpreted to either result from felsic metamorphism during an interval of rifting, or during the migration of magma along faults and cracks in the postrift period. VP/VS ratios occurring in concert with high VP anomalies in the continent‐ocean transition zone support a mafic composition of metapelitic granulite, which was either formed by magmatic intrusions or contact with mantle melting that stem from the upwelling of the asthenospheric mantle during the initial break‐up and onset of the seafloor spreading stage in the SCS.

Velocity Models for the Crust Hosting the Main Aftershock Cluster of the 2011 Mineral, Virginia, Earthquake

AbstractThree-dimensional P- and S-wave velocity (VP and VS) models are determined for the crust containing the main aftershock cluster of the 2011 Mineral, Virginia, earthquake using local earthquake tomography. The inversion uses a total of 5125 arrivals (2465 P- and 2660 S-wave arrivals) for 324 aftershocks recorded by 12 stations. The inversion volume (22 × 20 × 16 km) is completely contained within the Piedmont Chopawamsic metavolcanic terrane. The models are well resolved in the central portion of the inversion volume in the depth range 1–5 km; good resolution does not extend to the hypocenter depth of the mainshock. Most aftershocks are located within a northeast-trending, southeast-dipping region containing negative VP anomalies, positive VS anomalies, and VP/VS ratios as low as 1.53. These velocity results strongly argue for the presence of quartz-rich rocks, which we attribute to either the presence of a giant quartz vein system or metamorphosed orthoquarzite sandstones originally deposited on the Laurentian passive margin and subsequently incorporated into the Chopawamsic thrust sheets during island arc collision in the Taconic orogeny.

Orogenic lithosphere and slabs in the greater Alpine area – interpretations based on teleseismic P-wave tomography

Abstract. Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (∼ 180 km) European lithosphere. Our approach combines teleseismic P-wave tomography and existing local earthquake tomography (LET), allowing us to image the Alpine slabs and their connections with the overlying orogenic lithosphere at an unprecedented resolution. The images call into question the conventional notion that downward-moving lithosphere and slabs comprise only seismically fast lithosphere. We propose that the European lithosphere is heterogeneous, locally containing layered positive and negative Vp anomalies of up to 5 %–6 %. We attribute this layered heterogeneity to seismic anisotropy and/or compositional differences inherited from the Variscan and pre-Variscan orogenic cycles rather than to thermal anomalies. The lithosphere–asthenosphere boundary (LAB) of the European Plate therefore lies below the conventionally defined seismological LAB. In contrast, the lithosphere of the Adriatic Plate is thinner and has a lower boundary approximately at the base of strong positive Vp anomalies at 100–120 km. Horizontal and vertical tomographic slices reveal that beneath the central and western Alps, the European slab dips steeply to the south and southeast and is only locally still attached to the Alpine lithosphere. However, in the eastern Alps and Carpathians, this slab is completely detached from the orogenic crust and dips steeply to the north to northeast. This along-strike change in attachment coincides with an abrupt decrease in Moho depth below the Tauern Window, the Moho being underlain by a pronounced negative Vp anomaly that reaches eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted as representing hot upwelling asthenosphere that heated the overlying crust, allowing it to accommodate Neogene orogen-parallel lateral extrusion and thinning of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the east. A European origin of the northward-dipping, detached slab segment beneath the eastern Alps is likely since its down-dip length matches estimated Tertiary shortening in the eastern Alps accommodated by originally south-dipping subduction of European lithosphere. A slab anomaly beneath the Dinarides is of Adriatic origin and dips to the northeast. There is no evidence that this slab dips beneath the Alps. The slab anomaly beneath the Northern Apennines, also of Adriatic origin, hangs subvertically and is detached from the Apenninic orogenic crust and foreland. Except for its northernmost segment where it locally overlies the southern end of the European slab of the Alps, this slab is clearly separated from the latter by a broad zone of low Vp velocities located south of the Alpine slab beneath the Po Basin. Considered as a whole, the slabs of the Alpine chain are interpreted as highly attenuated, largely detached sheets of continental margin and Alpine Tethyan oceanic lithosphere that locally reach down to a slab graveyard in the mantle transition zone (MTZ).

Nested regional-global seismic tomography and precise earthquake relocation along the Hikurangi subduction zone, New Zealand

SUMMARY Seismic and geodetic examinations of the Hikurangi subduction zone (HSZ) indicate a remarkably diverse and complex system. Here, we investigate the 3-D P-wave velocity structure of the HSZ by applying an iterative, nested regional-global tomographic algorithm. The new model reveals enhanced details of seismic variations along the HSZ. We also relocate over 57 000 earthquakes using this newly developed 3-D model and then further improve the relative locations for 75 per cent of the seismicity using waveform cross-correlation. Double seismic zone characteristics, including occurrence, depth distribution and thickness change along the strike of the HSZ. An aseismic but fast Vp zone separates the upper and lower planes of seismicity in the southern and northern North Island. The upper plane of seismicity correlates with low Vp zones below the slab interface, indicating fluid-rich channels formed on top and/or within a dehydrated crust. A broad low Vp zone is resolved in the lower part of the subducting slab that could indicate hydrous mineral breakdown in the slab mantle. In the northern North Island and southern North Island, the lower plane of seismicity mostly correlates with the top of these low Vp zones. The comparison between the thermal model and the lower plane of seismicity in the northern North Island supports dehydration in the lower part of the slab. The mantle wedge of the Taupo volcanic zone (TVZ) is characterized by a low velocity zone underlying the volcanic front (fluid-driven partial melting), a fast velocity anomaly in the forearc mantle (a stagnant cold nose) and an underlying low velocity zone within the slab (fluids from dehydration). These arc-related anomalies are the strongest beneath the central TVZ with known extensive volcanism. The shallow seismicity (<40 km depth) correlates with geological terranes in the overlying plate. The aseismic impermeable terranes, such as the Rakaia terrane, may affect the fluid transport at the plate interface and seismicity in the overlying plate, which is consistent with previous studies. The deep slow slip events (25–60 km depths) mapped in the Kaimanawa, Manawatu and Kapiti regions coincide with low Vp anomalies. These new insights on the structure along the HSZ highlight the change in the locus of seismicity and dehydration at depth that is governed by significant variations in spatial and probably temporal attributes of subduction zone processes.

3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

AbstractBased on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.

Lithospheric Delamination Beneath the Southern Puna Plateau Resolved by Local Earthquake Tomography

AbstractWe present a local earthquake tomography to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. In the upper crust, prominent low Vp anomalies are found beneath the main volcanic centers, indicating the presence of magma and melt beneath the southern Puna plateau. Beneath the Moho at around 90‐km depth, a strong high Vp anomaly is detected just west of the giant backarc Cerro Galan ignimbrite caldera. This high Vp anomaly is only resolved if earthquakes with an azimuthal gap up to 300° are included in the inversion. However, we show through data subset and synthetic tests that the anomaly is robust due to our specific station‐event geometry and interpret it as a delaminated block of lower crust and uppermost mantle lithosphere under the southern Puna plateau. The low velocities in the crust are interpreted as a product of the delamination event that triggered the rise of fluids and melts into the crust and induced the high topography in this part of the plateau. The tomography also reveals the existence of low‐velocity anomalies that link arc magmatism at the Ojos del Salado volcanic center with slab seismicity clusters at depths of about 100 and 150 km and support fluid transport in the mantle wedge due to dehydration reaction within the subducted slab.

Three-dimensional seismic tomographic imaging beneath the Sea of Marmara: evidence for locked and creeping sections of the Main Marmara Fault

SUMMARYWe present 3-D Vp and Vp/Vs tomographic images for depths down to 22 km along with precise earthquake locations beneath the Sea of Marmara using the seismological data sets collected during the Ocean Bottom Seismometer (OBS) experiments conducted in 2001 and 2006. The unique data set collected by 52 OBSs and augmented by 14 land stations include 3852 P and 2643 S arrival times from 434 earthquakes and 4744 P arrival times from 557 air-gun shot records. From the tomographic depth sections and relocated seismicity, we identify two shear zones which are marked by narrow-width (∼10 km), low Vp (4.3–5.3 km s–1) and high Vp/Vs ratio (∼2.0) anomalies coinciding with the surface trace of the Main Marmara Fault (MMF) and extending near vertically from 8 km depth below the basins into the deeper part of the crust. We consider these shear zones to be the strike-slip deformation signature of the MMF at depth. The western shear zone imaged lies between the Western High and the eastern end of the Central Basin including the 60-km-long segment of the MMF and contains higher seismicity and is visible in the 8–22 km depth range. The eastern shear zone, coinciding with the NW–SE trending surface trace of the MMF, is imaged beneath the Çınarcık Basin with lower seismicity and tomographic resolution. Furthermore, these shear zones may contain fractures filled with deep crustal fluids allowing partial aseismic creep. Within these shear zones, we also identify small-scale, high Vp (6 km s–1) anomalies with low seismicity indicative of strong patches of the fault at depth that can be considered as asperities. Between these shear zones, our tomographic images clearly show a central zone that has homogenously high Vp (∼6.0–6.5 km s–1) and low Vp/Vs ratio (∼1.7) over a large area including the 50-km-long central segment of the MMF lying between the eastern end of the Central Basin and western end of the Çınarcık Basin. Thus, we infer that the central MMF is in a locked state based on its high shear strength, low seismicity and a lack of detectable geodetic slip. Furthermore, a large earthquake might not nucleate on the central MMF since it may be strongly locked at the current level of stress loading, and thus it may behave as a seismic barrier or anti-asperity. On the other hand, the central MMF is a good candidate for a supershear rupture because it has nearly uniform elastic properties and low background seismicity indicating the homogeneity of friction and pre-stress across the fault, which are presented as the main characteristics of supershear fault ruptures. From the seismic hazard perspective, we speculate that a large earthquake may be expected to nucleate within either of the imaged shear zones with the possibility of the central MMF acting as a seismic barrier to stop or to join the rupture in a supershear mode. The imaged features have important implications for various aspects of seismic hazard and crustal dynamics in the Sea of Marmara region.

Time‐evolving seismic tomography: The method and its application to the 1989 Loma Prieta and 2014 South Napa earthquake area, California

AbstractWe propose a time‐evolving approach to conduct traveltime seismic tomography in the 1989 Mw 6.9 Loma Prieta earthquake and 2014 Mw 6.0 South Napa earthquake area, California. The recording period of the chosen seismic data between 1 January 1967 and the day before the 2014 South Napa earthquake is divided into two time windows, separated by the 1989 Loma Prieta earthquake. In each time window the subsurface velocity structure is iteratively updated. Starting from the final model of the first time window, the velocity model has been successively improved throughout iterations in the second time window, indicating that the traveltime data of later time windows have provided extra information to refine the subsurface images. Strong heterogeneities are observed in the final P wave velocity model. Both of the two large earthquakes occurred at transition zones in between high VP and low VP anomalies. In all, this study shows the effectiveness of the time‐evolving seismic tomography method.

Array analyses of SmKS waves and the stratification of Earth’s outermost core

We perform array analyses of SmKS waves in order to investigate the Vp structure of the Earth’s outermost core. For earthquakes recorded by broadband seismometer networks in the world, we measure differential travel times between S3KS and S2KS, between S4KS and S3KS, and between S5KS and S3KS by array techniques. The differential times are well fit by a Vp model of the Earth’s outermost core, KHOMC (Kaneshima and Helffrich, 2013). Differential slownesses of S4KS and S2KS relative to S2KS are also measured for the highest quality data. The measured slownesses, with unique sensitivity to the outer core 200–400km below the CMB, are matched by KHOMC. These observations consolidate the evidence for the presence at the top of the outer core of a layer that has a distinctively steeper Vp gradient than the bulk of the outer core. We invert new SmKS differential time data set by a tau-p method and attempt to refine the Vp profile of KHOMC. The essential features of KHOMC are preserved after the model refinement. However, the newly estimated layer thickness is nearly 450km, which is thicker than that of KHOMC. The Vp anomalies relative to PREM for the depths 400–800km below the CMB are less than 0.03km/s, consistent with the degree of agreement between different Vp models for the depth range.

S−to−P heterogeneity ratio in the lower mantle and thermo‐chemical implications

AbstractWe evaluate the thermo‐chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the heterogeneity ratio ( ) as mapped in our model SPani and in alternative joint models. Robust structural differences between VP and VS evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between VP and VS that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of , making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, global median value ( ) drops from to , at 2500 km depth when the VP component of SPani is replaced by a VP model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani VP anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on and for the conversion of and into density further complicates a precise chemical interpretation.

Seismic velocity structure and earthquake relocation for the magmatic system beneath Long Valley Caldera, eastern California

A new three-dimensional (3-D) seismic velocity model and high-precision location catalog for earthquakes between 1984 and 2014 are presented for Long Valley Caldera and its adjacent fault zones in eastern California. The simul2000 tomography algorithm is applied to derive the 3-D Vp and Vp/Vs models using first-arrivals of 1004 composite earthquakes obtained from the original seismic data at the Northern California Earthquake Data Center. The resulting Vp model reflects geological structures and agrees with previous local tomographic studies. The simultaneously resolved Vp/Vs model is a major contribution of this study providing an important complement to the Vp model for the interpretation of structural heterogeneities and physical properties in the study area. The caldera is dominated by low Vp anomalies at shallow depths due to postcaldera fill. High Vp and low Vp/Vs values are resolved from the surface to ~3.4km depth beneath the center of the caldera, corresponding to the structural uplift of the Resurgent Dome. An aseismic body with low Vp and high Vp/Vs anomalies at 4.2–6.2km depth below the surface is consistent with the location of partial melt suggested by previous studies based on Vp models only and the inflation source locations based on geodetic modeling. The Sierran crystalline rocks outside the caldera are generally characterized with high Vp and low Vp/Vs values. The newly resolved velocity model improves absolute location accuracy for the seismicity in the study area and ultimately provides the basis for a high-precision earthquake catalog based on similar-event cluster analysis and waveform cross-correlation data. The fine-scale velocity structure and precise earthquake relocations are useful for investigating magma sources, seismicity and stress interaction and other seismological studies in Long Valley.

Mechanical properties and processes of deformation in shallow sedimentary rocks from subduction zones: An experimental study

AbstractTo better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura‐Boso paleo‐accretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as postexperiments P‐wave velocity (Vp) measurements. During experiments, deformation is principally noncoaxial and accommodated by two successive modes of deformation, both associated with strain‐hardening and velocity‐strengthening behavior: (1) compaction‐assisted shearing, distributed in a several mm‐wide shear zone and (2) faulting, localized within a few tens of μm‐wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Young's modulus all over the tests, (2) anomalously low Vp in the deformed samples compared to their porosity and (3) an increase in sensitivity of Vp to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized versus distributed) of shallow (z < a few km) sediments is mainly controlled by the variations in stress/strain rate during the seismic cycle and is therefore independent of the porosity of sediments. Finally, we show that the effect of strain, through cement breakage and microcracks formation, may lower Vp for effective pressure up to 40 MPa. As a consequence, the low Vp anomalies observed in Nankai accretionary prisms by seismic imaging between 2 and 4 km depth could reflect sediment deformation rather than porosity anomalies.

A 3D Vs model of the upper mantle beneath Italy: Insight on the geodynamics of central Mediterranean

We compute a high resolution Vs model of the upper mantle underneath Italy by inverting relative residuals of S-wave arrivals from teleseisms recorded at the Italian broad-band national seismic network. We also present a model of Vp/Vs perturbations obtained from similarly resolved P- and S-wave velocity models. The joint analyses of Vs and Vp/Vs models suggests that compositional variations concur with temperature variations to generate the observed velocity patterns, giving deeper insight into the upper mantle structure. The investigation of coherent high Vs and high Vp anomalies permits a comprehensive picture of the main oceanic domains involved in the puzzling subduction system of the central Mediterranean. Slab windows and tears developed in the Ionian slab are highlighted by regions of low Vp, low Vs and high Vp/Vs anomalies. The Adria continental lithosphere is characterized by high Vp, high Vs and low Vp/Vs anomalies that trace the sinking of eclogitized material underneath the Apennines and the Eastern Alps chains. The overall tomographic results yield the development of a new conceptual model for the Alps and Apennines formation, in which three basic processes occurred in different time and different locations all around the Alpine–Apennines system: continental collision, slab break-off and delamination of the upper-plate lithosphere.

Characterization of a stratigraphically constrained gas hydrate system along the western continental margin of Svalbard from ocean bottom seismometer data

The ongoing warming of bottom water in the Arctic region is anticipated to destabilize some of the gas hydrate present in shallow seafloor sediment, potentially causing the release of methane from dissociating hydrate into the ocean and the atmosphere. Ocean-bottom seismometer (OBS) experiments were conducted along the continental margin of western Svalbard to quantify the amount of methane present as hydrate or gas beneath the seabed. P- and S-wave velocities were modeled for five sites along the continental margin, using ray-trace forward modeling. Two southern sites were located in the vicinity of a 30 km long zone where methane gas bubbles escaping from the seafloor were observed during the cruise. The three remaining sites were located along an E-W orientated line in the north of the margin. At the deepest northern site, Vp anomalies indicate the presence of hydrate in the sediment immediately overlying a zone containing free gas up to 100-m thick. The acoustic impedance contrast between the two zones forms a bottom-simulating reflector (BSR) at approximately 195 m below the seabed. The two other sites within the gas hydrate stability zone (GHSZ) do not show the clear presence of a BSR or of gas hydrate. However, anomalously low Vp, indicating the presence of free gas, was modeled for both sites. The hydrate content was estimated from Vp and Vs, using effective-medium theory. At the deepest northern site, modeling suggests a pore-space hydrate concentration of 7–12%, if hydrate forms as part of a connected framework, and about 22% if it is pore-filling. At the two other northern sites, located between the deepest site and the landward limit of the GHSZ, we suggest that hydrate is present in the sediment as inclusions. Hydrate may be present in small quantities at these two sites (4–5%) of the pore space. The variation in lithology for the three sites indicated by high-resolution seismic profiles may control the distribution, concentration and formation of hydrate and free gas.

Evidence of large scale deformation patterns from GPS data in the Italian subduction boundary

We present the velocity field in Italy derived from over 300 continuous GPS stations operated in the 1998–2009 time span. The GPS network maps the whole country with a mean inter-site distance of about 50 km and provides a valuable source of data to study the ongoing deformation processes in the central Mediterranean. The estimated horizontal and vertical velocity fields show major significant features and also less known second-order kinematic features. A general uplift characterizes the whole Apennines and Alpine belts that follow the topographic ridge, whereas the Po Plain shows a gradually increasing subsidence from west to east. The Apennines belt displays a distinctive extension (50–80 10 − 9 yr − 1 ) while compressive tectonic regimes characterize northern Sicily, eastern Alps and the northeast front of the northern Apennines (25–50 10 − 9 yr − 1 ). Second-order deformation patterns, on large scale wavelength (~ 100 km) have been detected on the accretionary prism of central and southern Apennines that are highly correlated with other geophysical data (Vp anomalies, seismic anisotropy, etc.) and related to deep rooted sections (70–100 km), marked by different subduction regimes. Apparently at this scale-length the observed deformations are governed by the lithosphere as a whole. We interpret these deformations as a result of different subduction mechanisms, such as variations of the subduction rollback velocity affecting different segments of the subduction zone and/or to mantle flows in proximity of the slab edges. Further south, in central-southern Sicily, we detect a contraction of (− 1.1 ± 0.2) mm/yr that probably accommodates part of the Africa–Eurasia convergence on the outer thrust front of the Apennines–Maghrebides belt. This hypothesis agrees with an independent analysis of the seismicity associated to the Sicilian Basal Thrust, thought to be still active. The ITRF2005 estimates of the new GPS velocity field are available also in SINEX format as supplementary file S1.